Vacuum chamber lid - crazing on underside

[Mylowill]

Hi all,

I've been stabilising a number of pen blanks however noticed on the 2nd batch that the lid (18mm Acrylic) originally thought was cracked but after taking a closer look its only surface crazing on the underside or the vacuum side. It seems ok although not sure I want to trust it for ever. They radiate out from the fittings hole which came pre-drilled.

1) Has anyone had any experience of this or similar?

2) is it safe to continue using?

3) How can I stop this getting worse or when I replace, happening again?

Have tried to take a few photos and will try to attach or include in the post, although not sure how well it's come out or even if it's visible.

Was bought as a set inc. pump off of the ebay and was shipped from continental Europe to the U.K. I've had it roughly a year (so doubt I'll be able to get replacement lid...) but haven't used it much - this was probably only the 3rd time.

I'm using Cactus Juice and I had filled the pot pretty full using 3 internal containers stacked on each other with the top one also being coloured blue with alumilite dye. The lid wasn't touched by the cactus juice nor did any of the containers overflow. Have included a photo which I took during vacuum before the crazing happened.

Any suggestions or advice would be much appreciated.

Many Thanks,

Myles

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[Dieseldoc]

If it were me I would relocate the fitting to the side of the metal pot, as having a hole in the center of the lid will let the lid flex and I believe that is a weak point in the center of the lid which causing spider like cracks fro center out. You can get a solid glass 3/8 thick lid made for it. Just my two cents.

 

[robutacion]

Well, from the last pic, I have to ask, what type pump do you have...?
I'm referring to the oil gauge on the lid, is either the gauge isn't reading right or you pump is achieving far above the sea level of -30inHg, yours is at about -32inHg and I never seen that.

If the acrylic lid is too thin and the vacuum pump is pulling extreme vacuum, the holes on the lid are the week point and will be pushed in under full vacuum creating those spider cracks. In my view, you don't have to be afraid of it "exploding" if anything when in full vacuum the lid cracks more allowing air in and at such low pressures it won't hurt anyone, if it happens you will need to replace the lid with some thicker acrylic or tempered glass so don't panic, keep using the system.

Cheers
George

 

[MRDucks2]

I bought a used one with some cracks/crazing and still using it 3 years later. It may eventually start leaking but hasn't yet. I am guessing here but expect you haven't clipped the nipple, removed the seal or pressed the brass stopper on your vacuum gauge (depend on how it is made). That is likely why is isn't reading correctly.

 

[Mylowill]

Thanks for the responses....
yep hadn't removed the seal
I'm at sea level so what should it read?!
Seems to still go to -30hg Although can't imagine it's very accurate - wasn't massively expensive!


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[howsitwork]

I go along with the safety in use above but have had an acrylic lid fracture on me, A bum clenching moment for sure

It does look like stress lines in your case. In mine it was the resin ( stickfast) attacking both the lid and the cured araldite resin used to seal the pressure gauge etc in place. I then made a replacement in 12mm polycarbonate having tested that by leaving pieces in resin for days then hitting with hammer, no issues found so made my new lid.

No issues now.

 

[robutacion]

Thanks for the responses....
yep hadn't removed the seal
I'm at sea level so what should it read?!
Seems to still go to -30hg Although can't imagine it's very accurate - wasn't massively expensive!


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Those vacuum gauges filled with oil are normally very accurate, the reason the gauge only shows numbers up -30inHg is because that is what vacuum pressures can be achieved at sea level, any elevation after that and that number will start to decrease, I'm at 270 meters elevation, the max I can achieve in just of 29inHg.

Cheers
George

 

[egnald]

Greetings from Nebraska!

I had something very similar happen to me recently. I bought a 3-Gallon turnkey rig from eBay, but on mine, actual cracks formed on the lid originating at the hole. Then, my lid imploded which I also assumed is what messed up my gauge (it started reading way more than the vacuum that is possible - kind of like yours).

My ultimate solution was to follow many of the outstanding recommendations from other members in the forum. I bought a new, polycarbonate (Lexan) lid Acrylic typically means plexiglass which isn't nearly as robust as polycarbonate for this kind of application. I also mounted the plumbing through the top of the stainless steel pot rather than compromising the integrity of the lid. Then I replaced the gauge with a little larger and better quality one. And, finally, I broke down and replaced the lid again with one made out of glass. I now have a system that works and I am more comfortable being around when it is operating.

Here is a link to the thread so you can see some of the recommendations I received from the other IAP members.

First Time Stabilizing - The Excitement is Mounting!

Greetings from Nebraska, The title pretty much says it all - I have my trial batch of blanks (Redwood and Walnut) in my new stabilizing rig under vacuum with Cactus Juice. Prior I had dried them in my toaster oven at 220F for about 24 hours, then vacuum sealed them in bags while they were still...

www.penturners.org

Good Luck and Best Regards,
Dave

 

[Mylowill]

Thanks for all the responses and information which has definitely reduced my original concern - very much appreciated.

Dave - my set-up is exactly the same as yours, why am I surprised.

Going to monitor, at the moment they are only on the surface but assume they are going to get worse and start looking for a good quality replacement.

Again thanks all


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[egnald]

Thanks for all the responses and information which has definitely reduced my original concern - very much appreciated.

Dave - my set-up is exactly the same as yours, why am I surprised.

Going to monitor, at the moment they are only on the surface but assume they are going to get worse and start looking for a good quality replacement.

Again thanks all

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FYI - in my thread, user Pensmythe included a link and recommendation for a glass lid from shopbvv along with a link to their site. Of all of the polycarbonate and other stuff I bought for replacements, the glass one Pensmythe recommended was the least expensive and the one that I use and feel the most comfortable with. (After an implosion, even the little bit of crazing that started to show up on my polycarbonate made me nervous enough that drove me to the glass lid.) - Good Luck - Dave

 

[howsitwork]

Well ride comes before a fall, so despite my earlier comments on the forum about polycarbonate being immune to stickfast resin and not crazing under vacuum , tonight having left it overnight at -29 on high pressure. vacuum the damn lid shows minor crazing on the underside, estimate cracks are 2mm deep on a 12 mm thick lid.

I think it was the higher vacuum pressure that did it but there was some splashing on the lid underside as I let it get fairly vigorous at first. Usually I have. filter set up between it and the pump which limits the pressure to -28Hg but last night I took a short cut and connected direct to pump.

So more care next time and I'm cross with myself but just wanted to let everyone know !

I did test extensively with the resin and there doesn't seem to be any etching lines where there are splashes so think that's fine, just over sucked as it were

 

[egnald]

Greetings,

I've read some very interesting things lately about stress cracks in polycarbonate. One of the papers talked a lot about exposure to certain chemical solvents that can affect the polymers and their strength which ultimately present themselves as shallow surface cracks but only when some kind of stress is applied. The aging process under the same conditions is that the cracks expose fresh polymers that are in-turn affected by additional exposure and deepen when stress is applied... so the process repeats. I don't know for sure whether that has any impact on the cracks that are in the photos above (or in the one's that formed on my polycarbonate lid). The paper definitely indicated that both conditions are necessary to make the cracks appear and that under normal circumstances with one or the other by themselves stress crack formation is very unlikely.

Regards,
Dave